Integrated circuit chip modular assemblies are being developed for use in computer work stations and other similar equipment where the bond pads on the surface of each chip are directly interconnected to circuitry on thin film multilayer membranes. Such multilayer membranes are composed of a layer of polyamide and a layer of copper on the polyamide with circuitry etched into the copper. Gold plated contact surfaces are arranged on the copper circuitry for mating with bond pads on the chips, the contact surfaces and bond pads having identical pattern layouts. The surface of the membrane is coated with a photo sensitive polymer, masked, and developed to form boundary structures or nests for receiving the chips. The nests have features that very precisely locate the chips with respect to the contact surfaces so that the bond pads are in alignment therewith. Each of the bond pads of the chips has a small gold contact ball attached thereto for contacting its respective contact pad on the membrane. Each gold ball is formed by wire bonding a soft gold wire to the bond pad in the usual manner and then breaking the wire off near the bond joint leaving a short stub. The stub is then formed into a ball, or other desired shape, and a desired height above the surface of the chip by mechanical means such as coining or by other suitable means. The membrane is held taut in a rectangular frame. The chips are positioned within their respective nests on the membrane and backed up on their opposite surfaces with a heat sink in the usual manner. A somewhat soft rubber member, about the same size as the membrane, is positioned against the side of the membrane opposite the chips. A pressure plate urges the rubber member against the membrane so that the contact surfaces are pressed against their respective gold contact balls on the chips with a force that is proportional to the hardness of the rubber member. The hardness is chosen to provide a desired contact force. As the rubber member is pressed against the membrane, however, relatively large tensile stresses are imposed on the membrane. This occurs because the soft rubber member and the membrane will deform around the contact balls while the remaining area of the rubber member pushes against the taut surface of the membrane. This perpendicular loading of the taut surface of the membrane results in substantial tensile forces in the direction of the plane of the membrane which will either break the bond between the membrane and the frame or will result in other dimensional distortions that may affect the reliability of the product.
What is needed is an integrated circuit chip module assembly having a contact pressure equalizer that applies a desired amount of force to each contact without resulting in adverse tensile forces in the membrane.